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1
525
561
605635
641
Optical Fiber
Transmission
Media
--:]TER
OUTLINE.:::.rduction
-:.tor)
of
Optical Fiber Communications
-,:;ical
Fibers
versus
Metallic
Cable
Facilities
-
:;tromagnetic
Spectrum
:
..k
Diagram
of
an
Optical
Fiber
-- -
:'nmunications
System
::r.al
Fiber
Types
'
-r:ht
Propagation
I
E
Optical Fiber Configurations
l-9
Optical Fiber Classifications
I l0
Losses
in
Optical Fiber
Cables
I ll
Light
Sources
1-
ll
Optical
Sources
I l-l
Light
Detectors
l-
1-1
Lasers
I
'
I
5
Optical
Fiber
System
Link
Bud-set
-:
-::TIVES
|
-.-.'-rc
optical
comnrunications
I
:-:renr
an
overview
ofthe
history
ofoptical
tibers
and
optical
fiber
communications
I
:-.pare
the advantages and disadvantages
of optical fibers
over
metallic
cables
:::ne
electromagnetic
frcquency
and
wavelenqth spectrunt
I
-
:.:nbe
several types
of optical fiber construction
t
:
,
:.,iin
the
physics
of
light
and the
following
terms:
velocity of
propagation.
refraction. refractir
e
index.
critical
II
--:-e.
acceptance
angle,
acceptance
cone. and
numerical
aperture
-:.-ribe
how
light
waves propagate
through
an
optical fiber
cable
-.-..te
ntocles
of
propugtttion
and irtdex
profile:
t. ribe
the
three types
of optical fiber configurations:
single-mode
step
index. multimode
step
index.
and
mul-
'Je
-craded
index
:..:rbe
the various
losses
incurred in optical fiber
cables
-:
:e
liqht
source
and
optical
power
-:i.rbe
lhe
following light
sources:
tight-emitting
diodes and
injection
diodes
- ..-:rbe
the
following light
detectors:
PIN
diodes and avalanche photodiodes
:.-::be
the
operation
ofa
laser
.:..:n
ho$
to calculate
a
link
budget
for
an
optical
fiber
system
1
II II
a
1.1
INTRODUCTION
1-2
HISTORY
OF
Optical fiber
cables
are
the newest and
probably
the most
promising
type
of
guided trans-mission
medium for
virtually
all
forms
ot'digital
and data
communications applications'
in
cluding local, metropolitan'
and
wide
area
networks With
optical
fibers'
electromagneticwaves are guided
thiough
a
media composed
of
a
transparent
material
without
using elec-
trical cuneirt
t'low.
With
optical fibers, electromagnetic
light
waves propagate
through
the
media
in
much the
same
way
that
radio
signals propagate
through Earth's
atmosphere'
In
essence,
an
oPtic'(tl
cotttttlLotic4'i;rts
J-)
ste''
is one that
uses
light
as
the
carier
of
information.
Propagating
light
waves
thrcugh Earth's
atmosphere
is
difficult
and often
im-
prr.tl.ul.
Cun. quJntly.-opiical fiber communications
systems
use
glass
or plastic
fiber
ca-Lles
to
'colralri:
the
light
waves and
guide them
in
a
manner
similar
to
the
way electro-
magnetic \i'aves
are
guidecl
through
a
metallic
transmission
medium
.
The
itdbrnutiort-carning cayrcin of
any
electronic communications
system is
di-
rectly proportional
to
bandwidth. Optical fiber
cables
have'
for all
practical purposes'
an
in-
nnite
ilaniwidth.
Therefore. they
have the
capacity to carry much more
information
than
their metallic
counterparts or.
for
that
matter, even
tAe
most
sophisticated
wireless com-munications
sYStems.
For comparison purposes.
it
is
common to
express
the
bandwidth
of
an
analog
com-munications
system
as
a Percenlage
of
its
carier
frequency This
is
sometimes
called
the
bandtidth
utiiizatio,?
r4ti;.
For inatance.
a
VHF
raclio
communications
system operating
at
acarrierfrequencyofl00MHzwithl0-MHzbandwidthhasabandwidthutiliZationratioof
10olr.
A microwave rcdio
system operating
at
a
carrier frequency
of l0
GHz with a
l07rhandwirlth utilization ratio would
hive
I
GHz
of
bandwidth available Obviously'
the
higher
the
caniel
fiequency.
the
more
bandwidth available'
and the greater the
information-
ca-rrvins
caoacitl.
Lighr
frequencies
used
in
optical fiber communications
systems are be-
* i,i
io''
ir, no+
.l0'rHz(100.000GHzto400,000GHz)
A bandwidth
utiliza-
tion ratio
of
107.
would
be
a
bandwidth between
10,000
GHz
and
40'000
GHz'OPTICAL
FIBEB COMMUNICATIONSIn
1880.
Alexander
Graham
Bell
experimented
u
ith
an
appalatus he called
a
photophone'
The photophone was
a
device constructed t'rom
mirrors
and selenium detectors that
fians-
,oitt.,i
,ornd
*u.'.,
over
a
beam
of
light
The photophone
was
awkward
and
unreliable
and
hacl
no
real
practical application.
Actuall\.
Iisual light
was
a
primary
means
ofcommuni-
cating long
;efore
eleciionic communications
came
about
Smoke signals
and
minors
were
ur.d-ag.r-ago
to
conrel
shon.
simple
messages
Bell's
contraption' however'
was
the
tirst
attempt
at
using
a
beam
of
light for
carrying information'Transmission
of light
waves
for
any
useful
distance
through Earth's
atmosphere
is
impractical
because
water
vapor,
oxygen.
and
particulates
in
the air
absorb and
attenuate
the signals
at
light
frequencie.s.
Consequently. the
only practical
type
of optical communi-
catiois
system is one that
uses
a
fiber
guide
ln
1930' J'
L.
Baird'
an
English scientist'
and
c. w.
Hansell. a scientist
from
the
u;ircd
States,
were
granted patents
for
scanning
and
transmitting television
images
through
uncoated
fiber cables
A
few
years
later'
a
German
scientist
named
H.
Lamm successfully transmitted
images
through
a single glass
fiber
At
that
time,
most people considered
fiber
optics more
of
a
toy
or
a
laboratory
stunt and' con-sequently.
it
was
not
until
the
early
1950s
that
any substantial
breakthrough
was'.qnade
in
the
field
of tiber
oPtics.
In
t951.
A.
b.
S. van
Heel
of
Holland
and
H H
Hopkins
andN'
S
Kapany
ofEn-
glandexperimentedwithlighttransmissionthroughDundlesoffibers.Theirstudies]edto
t-h.
d.r iop..rt
of
the
fle;ible fberscope,
which
is
used
extensively
in
the
medical
held'It
uas Kapany who
coined the
teIm
fiber
optics
in
1956'
Chapter
1
ns-
in-
:ric
rhe
iof
im-
ln
1958.
Charles
H.
Townes.
an
American.
and
Afthur
L.
Scharvlou. a
Canadian.wrote
a
paper
describing
how
it
was
possible
to
use
stimulated emission
for amplifying
light
waves (laser)
as
well
as
microwaves
(maser).
Two
years
later. Thcodore
H. Maintan.
a
sci
entist
with
Hughes
Aircraft
Company,
built
the
first
optical
maser
The laser
(Iight amplification
by
.rtimulated emission
of
radiatir)n)
was
invented
in
1960.
The laser's
relatively high output po*'er. high tiequcncy of
operation.
and
capabilitlof carrying
an
extremely
wide
bandwidth signal
make
it
ideally
suited
for
high-capacitycommunications
systems.
The
invention of
the
laser
-sreatly accelerated
research
efforts infiber-optic coinmunications. although
it
was
not
until
I967
that
K. C.
Kao
and
C. A. Bock-
hanr
of
the Standard
Telecommunications Laboratory in
England proposed
a
new
conrmu-nications medium
using
c
larlded
fiber
cables.
The fiber
cables
available
in
the
1960s
were
extrenre)y
1tr.r.ir'
(more
than
1000
dB/km), which limited optical
transmissions to
short
distances.
ln
1970.
Kapron.
Keck. and
Maurer
of
Corning
Glass
Works
in
Corning, New
York.
developed an
optical
tiber
with
losses less
than
2
dB/km. That
was
the
big
breakthrough
needed
to
pcrnlit
practical
flber
optics communications
systems.
Since
l9?0, fiber
optics technology
has
grown
exponen-
tially.
Recently.
Bell
Laboratories
succ'essfully
transmitted
I
billion
bps
thlough
a
fiber
ca-
ble
for 600
miles
without
a
regenerator
In the
late
1970s and
early
1980s.
the
refinement
ofoptical
cables and the
development
ofhigh-quality,
affordable
light
sources and detectors opened the
door to
the
development
of
high-quality, high-capacity, etficient,
and
affordable optical fiber communications
systems.
By
the
late
1980s,
losses
in optical fibers
were reduced to
as
low
as
0.16
dB/krn.
and
in
1988
NEC
Corporation
set a
new long-haul transmission record
by
transrnitting I0
-uigabytes per second
over
80.1
kilometers
ofoptical fiber
Also
in
1988,
the
American
National
Standards
Institute
(ANSI)
published
th
e
St
trchntnous
Opricdl
Nenrork
(.SON
ET). By
the
mid-
I
990s.
opticnl voice
and
data
networks
were
commonplace throughout
the
United
States
and
much
ofthe world.
ca-IIO-
di-
L
in-
han
rll1-
)In-
.
the
lg
at
-atio
l0
,q(
the
:ion-
:
be-
iiza-
lt)tle.
rans-
:
and
nuni-
\\ ere
|
first
rre
isnuate
nuni-
r.
and
i
and
,I11AN
er.
At
.
con-rde
in
ri
En-led
tofield.
OPTICAL FIBERS VERSUS
CABLE
FACILITIESCommunications through
glass
or
plastic fibers
has several advantages
ovel
conven-tional metallic transmission
media
for both telecommunication
and
computer rretworkingapplications.
1-3-1
Advantages
of
Optical Fiber
Cables
The advantagcs
of
using
optical fibers include
the
tbllou
ing:
l.
Wider
bandridtlt
and
grcdter
iDformLltiott
('lPttit\'.
Optical
fiberr
hirr
e
treater in-formation
capacity than
metallic
cabies
becalrse
of
lhe
inherentl)
s
idel
bands
idth:
lr
ail-able
with optical
t'requencies.
Optical libers
ure
arailable
\\ith
band\\idlh\
up
lo
\e\eral
thousand
gigahertz.
The
pri,ran
eleclritttl
tottslunrs (lesi\tan -e.
inductance. and capaci-tance)
in
metallic
cables
cause
them
to
act
like
lo\\
-prss
iille[s.
$
hich
lintit
iheir
triir]\nlis-
sion
frequencies,
band$,idth.
bit
rate.
and
intbrmttion-carq ing
clpircil). \lode:n
opticalfiber communications
systems arc
capable
of transmitting
ser
elal
gigrbitr
per
second
over
hundreds
of
miles,
allowing literally millions
of
indi\
idLral
\
oice
.1nd clata
channels
to
be
combined
and
propagated over one
optical
tiber
cable.
2.
Inmwtitv
to
<rossr4lt. Optical fiber
cables
are
inmune
to crosstalk
becaLlse
glassand
plastic fibers
are
nonconductors
ofelectrical
curent.
Therelbre.
fiber
cables
are
not sur
rounded
by
a
changing magnetic
tield. which
is the
prima4
cause
ol
crosstalk
between
metallic
conductors located
physical)y
close to
each
other.
3.
lmmufiit\'tu stciic
interferefice.
Because
optical tiber
cables are
nonconductors
of
electrical current, they are
immune
to
static
noise due
to
electromagnetic
interference
(EMI)
caused
by
lightning,
electric motors.
relays.
fluorescent lights.
and
other electricalFiber
Transmission Media
noise sources (most
of
which
are
man-made).
For
the
same
reason'
fiber
cables
do
not ra-diate electromrg.netic
energl
4.
Entirotlma
t.tl
inrlrlnin.
Optical
fiber
cables are
more
resistant
to environmen-
ul
extremes
(including
weather
variations)
than
metallic cables Optical
cables also oper-
ate
over
a
wider
temperature range and
are less
aftected
by corrosive liquids
and
gases'
5.
Sa/en
anrl contefiien('e. Oplicdl
fiber
cables are
sat'er
and easier
to
install
and
maintain than
metallic
cables. Because glass and
plastic
fibers
are
noncondrrdors- there
are
no
electrical
currents
or
voltages associated
with
them
Optical fibers
can be used around
volatile
liquids
and
gasses
without
worying
about
their
causing explosions
or
fires
Opti-cal tibers
are also
smaller
and
much more
lightweight
and compact
than
metallic
cables'
Consequently.
they
are
more
f-lexible. easier
to
work
with'
require less
storage
space'
cheaper
to
transport.
and
easier
to install
and
maintain.
6.
Lrnter
trctnsmi.isiorr
/oss Optical libers
have
considerably
less
signal
loss than
their metallic
counterparts.
Optical tibers
are
cuffently
being manufactured
with
as
lit-
tle
as
a
few
tenths-of-a-decibel loss per
kilometer.
Consequently'
optical
regenerators
anit
amplifiers
can be spaced
considerably farther
apart
than
$ith
metallic
transmission
lines.
7.
Secrrill.
Optical
fiber
cables are
more
secure
than
metallic
cables
lt
is
virtuall)
impossible
to
tap
into
a
fiber
cable
without
the
user's
knowledge'
and
optical
cables
cannot
be
detected
with
metal detectors unless they
are
reintbrced
with
steel
for
strength'
8.
Durat:tilitl
(tnd
rcliabilitt
Optical fiber
cables
last longer
and are
more reliable
than
metallic
facilities
because
fiber
cables have
a
higher tolemnce
to
changes in
environ-
mental
conditions
and are
immune
to
colrosive
materials'
9.
Econontics. The cost
of optical fiber
cables is
approximately
the
same
as
metalli'
cables.
Fiber
cables have
less
loss and
require fewer
repeaters'
\
'hich
equates
to lower in-stallation
and
overall systen
costs and
improved
reliability'
1-3-2
Disadvantages
of
Optical Fiber
Cables
Although
the
advantag;s
of
optical
tiber
cables
far
exceed the
disadvantages
it
is impor-tant to
know
rhe
limirations of
the fiber. The disad antages
of optical
fibeIS include
Ihe
following:
l.
lntetf(kittg
cost.t
Optical fiber
cable
s)
stems are
virtually
useless
by themseh
e
Tobepracticalanduseful.the},muslbeconnectedtostandardelectronicfacilities.whic]:
often
require
expensir
e interf'aces
2.
Strengih.
Optical ilbers
bl
themsehes have
a
significantly
lower tensile
sffensti
than
coaxial
cabie.
This
can be
improred
by coating
the
fiber with
standard
Ker'lar
and
protective jacket
of
PVC.
In
addition.
glass
fiber
is
much more
tiagile
than copper
\\
iri'
making
fiber
less
attractive
where hardwarc
portability
is
required'
i.
Renu)te
electrical
por|er
Occasionally.
it
is necessary
to
provide electrical
po*
e:
to remote
interface
or regenerating equipment.
This
cannot
be
accomplished
with
the
opt:'
cal cable.
so
additional metallic
cables must be
included in
the cable assembly'
4.
OptiutlJiber
utbles
are more susceptible
to
Losses
iriroducetl
by bending
tlte c':'
b1c.
Electromagnetic
waves propagate
through
an
optical
cable
by either refraction or re'flection. Thereibre.
bending the cable
causes
irregularities
in
the cable
dimensions'
rcsu::-
ing
in
a
loss
of
signal power.
Optical fibers
are
also more prone
to manufacturing defec:'
as
even the most
minor
detect can
cause
excessive loss
of
signal
power'
5,
Speciali:ed
kx
s.
equiltnent.
trnd
truining' Optical
fibcr
cables
require
spec:'turls
to
splice anrl repair cables and special test equipment
to
make
routine
measuremen:'
Not
only
is
repairing fiber
cables
difficult
and expensive, but technicians
working
on
op:--
cal cables also require special
skills
and
training.
tn
addition'
sometimes
it
is
difTicult
to
-
cate
taults in optical
cables
because
there is no
electrical continuity'
Chapter'l
:l
i
i'l
j

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